The present invention relates to thin film antireflection coatings, and more particularly to an electrically-conductive, antireflection coating which attenuates light.
Certain articles are fabricated to attenuate light for various reasons such as heat reduction, eye protection and an improved visibility. These articles may also require an antireflection coating on at least one surface thereof. Such articles include sunglasses, solar control glazings and contrast enhancement filters.
In sunglasses, light attenuation protects the eye from bright light, and the antireflection coating reduces reflected glare from the surface of the lens facing the eye. Antireflection properties for sunglasses are usually provided by a multilayer coating comprising vacuum-deposited, transparent, dielectric films. The light attenuation feature may be an intrinsic property of the lens. This feature may also be introduced extrinsically by dying the lens. Preferably, sunglasses attenuate about 90 percent of the visible light.
Solar control glazings attenuate solar energy transmitted to the interior of, for example, a vehicle or building. These glazings preferably have a low reflection treatment on their interior surface to reduce distracting reflections. Low emissivity (E) coatings may be used to reduce reflections. Light attenuation for solar control glazings may be achieved by using a light absorbing glass. Light attenuation may also be provided by a vacuum-deposited, metal film or by a plastic sheet coated with a metal film and attached to the glazing by a suitable adhesive. The light attenuation is about 50 percent of the visible light.
A contrast enhancement filter is often used to enhance image contrast and reduce glare from the screen of a video display terminal (VDT). This filter is located between the VDT operator and the screen. Contrast enhancement filters may be made from light absorbing glass. The glass may transmit about 30 percent of incident light. Light from extraneous sources, such as windows and light fixtures, passes through the filter and is attenuated before it is reflected from the screen. After it is reflected from the screen, it must again pass through the filter before it is observed by the operator. After the second pass, light may be attenuated to about 10 percent of the intensity that it would have had without the filter. If reflectivity of the screen is about 4 percent, the images of extraneous light sources and objects may be reduced by more than 99.5 percent.
Light from the screen image passes through the contrast enhancement filter only once. Thus, it may be attenuated only about 70 percent. As such, visibility of the image is enhanced. A contrast enhancement filter is effective only if its outer facing surface is provided with an antireflection treatment. Preferably, both its inner and outer surfaces should be so treated. The antireflection treatment may comprise a multilayer, antireflection coating. Such a coating may have a perceived reflectivity for visible light, usually called the photopic reflection or the photopic reflectivity, less than about 0.25 percent. Filters having a photopic reflectivity of less than about 0.15 percent are preferable.
A contrast enhancement filter may be made from glass or plastic. If the filter is located close to the cathode ray tube (CRT), it may build-up static charges. Thus, one or both surfaces of the filter is preferably electrically-conductive and grounded to prevent the build-up of static charges. If the filter surfaces are provided with a multilayer, antireflection coating, electrical conductivity may be an intrinsic property of the coating. Electrically-conductive, transparent films, such as indium tin oxide, may be used in such coatings.
The cost of an electrically-conductive filter may be as great as about 30 percent of the cost of the VDT. The high cost of these filters can discourage their use.
It is well known that light-absorbing films may be used to construct antireflection layer systems. The simplest light absorbing systems include a low reflectivity metal film, such as chromium or molybdenum, in contact with a glass or plastic substrate, and a layer of a transparent dielectric material, such as magnesium fluoride or silicon dioxide, in contact with the low reflectivity film. These metal films may be very thin, on the order of about 5 nanometers (nm). The optical properties of such thin films are difficult to control as the metals tend to oxidize during the initial part of the deposition process. Subsequent oxidation or corrosion of the coating may also occur. A thin metal film may also provide inadequate electrical conductivity and only about 40 percent attenuation of visible light.
FIG. 1 shows the computed transmission (curve A) and reflection (curve B) values of a two layer system comprising a chromium film about 1.6 nm thick and a silicon dioxide film about 75.4 nm thick. The films are disposed on a glass substrate having a refractive index of about 1.52. The photopic reflection of the system is about 0.35 percent when observed from the side of the system opposite the substrate, i.e. from the air side of the system. The photopic transmission is about 75 percent.
Another antireflection system is a low E coating including a silver film having a high refractive index and bounded on either side by a dielectric film. The lowest reflection is obtained with relatively thin films of silver, for example 6 to 8 nm thick. Attenuation of visible light, however, is negligible.
The silver-dielectric layer system may be extended to include one additional silver film. This may increase the system's electrical conductivity and improve its antireflection performance. The silver films may be separated by a relatively high refractive index dielectric material having an optical thickness of about one-half wavelength at a wavelength of about 510 nm, which is about the middle of the visible spectrum. Each silver film will also be bounded by a layer of dielectric material. Each dielectric layer will have a refractive index of about one-quarter wavelength at a wavelength of about 510 nm.
This system is similar in function to the light-transmitting, heat-reflecting coating described in U.S. Pat. No. 4,799,745. The silver films of this coating must be relatively thin to provide the lowest possible reflection. Attenuation of visible light for this coating is one the order of about 10 percent. Sheet resistance may be about ten ohms per square, providing adequate electrical conductivity for most purposes.
FIG. 2 illustrates the transmission (curve C) and reflection (curve D) values for a system comprising two silver films and three dielectric layers. The system is deposited on a glass substrate. The layer sequence and physical thickness, beginning from the substrate, are as follows: zinc oxide (45.7 nm), silver (6.9 nm), zinc oxide (85.3 nm), silver (18.4 nm), and zinc oxide (43.3 nm). The refractive index of the glass substrate is 1.52.
Systems using combinations of a high light absorbing metal, such as chromium, and a low light absorbing metal, such as silver or gold, may also be constructed. Such combinations permit different values of photopic transmission while still providing relatively low reflection from at least one surface. In general, however, systems including a thin soft metal film, such as silver, gold or copper, have poor scratch resistance. Systems including thin films of silver or copper are also vulnerable to corrosion and may deteriorate within a few months when used on an unprotected surface.
The above-described layer systems may produce any one of the following: (1) high electrical conductivity and low reflection, (2) adequate light attenuation and low reflection, or (3) adequate light attenuation and high electrical conductivity. These systems do not provide a single structure which has high electrical conductivity, low reflection and adequate light attenuation.
As such, it is an object of the present invention to provide an electrically-conductive, antireflection layer system that provides a wide range of attenuation values for visible light, while still providing low photopic reflection.
It is a further object of the present invention to provide a light attenuating, antireflection layer system which may have a sheet resistance less than about 100 ohms per square.
It is yet another object of the present invention to provide an electrically-conductive, light attenuating, antireflection layer system which is abrasion and corrosion resistant.
It is also an object of the present invention to provide a corrosion resistant, abrasion resistant, electrically-conductive, adequate light attenuating, antireflection system which may be deposited by DC reactive sputtering in an in-line coating machine of the type used for architectural glass coating.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and the advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the claims.